Soil Use and Management (1999) 15,144^149
Relationships between land use, fertility and Andisol
behaviour: examples from volcanic islands
S. Perret
1& M. Dorel
Abstract. Soils developed on volcanic parent materials have many intrinsic qualities favourable to cropping. However, fertility decreases dramatically when they are badly managed. A short review and case studies from Re¨union and Guadeloupe highlight the special characteristics of these soils, and their response to management. The interplay of cropping systems and physical characteristics of Andisols is first considered through the example of Pelargonium and food crop systems in Re¨union. Progressive decrease in production and cropping potential shows in falling yields as well as in the overall decline of the system.The example of banana production in Guadeloupe highlights the increase in inputs needed to realise the land's potential and to maintain yields, in particular more tillage and pest treatment.
In both cases, these trends are connected to the co-evolution of soil characteristics and cropping systems.They lead to an increase of risks with less security and less scope in the choice of cropping systems.Technical solutions in the form of erosion-control measures, rotation and planting techniques have been developed and prove to be relevant and consistent in their benefit.
Keywords: Andisols, cropping systems, soil fertility, Pelargonium, bananas, Re¨union, Guadeloupe
I NTRODUCTION
A
ndisols are found mostly in volcanic zones at high altitudes and are widespread throughout the world in these environments.Initially, they are loose, friable, and often rich in nutrients, and support a wide variety of production systems. However, as they are often sloping and subject to high rainfall, they may suffer erosion and deterioration of the cultivation layer if poorly managed.
The purpose of this paper is to show how land use seems to be a function of soil behaviour and vice versa.
AN DISOLS AN D FAR M I NG SYSTEM S I N TH E FRANCOPHON E WORLD Geography, pedogenesis and the majorcharacteristics
Soils resulting from the break-down and weathering of basic volcanic material cover only small surface areas on a global scale, but they are very widely distributed, especially in wet inter-tropical areas. Being friable and loose to a good depth, free from stones, and well supplied with nutrients, they are usually densely settled. They occupy upland and piedmont zones (East Africa, Cameroon, Central America, Mexico and the Andes) and cover most of certain islands in the West Indies, Indian Ocean, South Pacific, Indonesia and Melane-sia (Quantin, 1972), including the French Overseas
Depart-ments of Re¨union, Guadeloupe and Martinique.
Soils with andic behaviour and properties can develop from massive basaltic lavas under heavy rainfall conditions, and more rarely from igneous rocks under certain geochem-ical conditions (Kimble, 1998). However, they mainly result from the fastweathering of fine basaltic, andesitic or trachytic ashes, and recent pyroclastic deposits, according to two major weathering processes: hydrolysis of volcanic glass and complexing by organic acids.
According to altitude and climate, the same soil sequence is observed on these parent materials in many tropical island settings: the Andisols (Andepts) exhibit a dominant halloysi-tic character at low altitudes (1200 mm rainfall), and then become allophanic then gibbsitic with increasing annual rainfall (Colmet-Daage & Lagache, 1965; Quantin, 1972; Zebrowski, 1975). Table 1 summarizes some physical and chemical data from an Andisol in Re¨union and in Guadeloupe.
In the field, the Andisols show two main horizons. One can easily identify the dark A horizon, overlying reddish brown or yellowish brown B horizons. Because of the nature of the ashfall, the horizons are generally horizontal and the transi-tion from A to B horizon is distinct.
Andisols generally have a granular structure in the A hori-zons due to disturbances of root mat and soil fauna, enhanced by the accumulation of organic matter (5 to 20% o.m.).The B horizons typically feel loamy and `puffy', due to low density and to high allophane and water content, and they still con-tain an important amount of humus (1 to 5% o.m.), despite their lighter colour. In the field, B horizons appear massive, without aggregates, but rich in microporosity. However, weak subangular blocks can easily be excavated.
Under forest and natural vegetation or on slight slopes, A horizons are usually thick (20 to 60 cm depth). Under
crop-1Department of Agricultural Economics, Extension and Rural Development,
Faculty of Biological and Agricultural Sciences, University of Pretoria, 0002 ^ Pretoria, SouthAfrica. Fax. 27-117065620. E-mail: [email protected] CIRAD, Centre de Coope¨ration Internationale en Recherche Agronomique pour le De¨veloppement
ping systems, erosion by water is so severe that few relict pro-files remain. On steep slopes, nearly all tilled soils have lost their A horizons, leaving B horizons exposed to desiccation.
These soils present unique physical, mechanical or water-related properties, that are subject to rapid change under cropping systems. The wide diversity of production systems implemented on these soils is also a characteristic.
Andisols and the cropping systems: two case studies
In Re¨union, andic soils cover half of the total surface area (Raunet,1991), and constitute 80% of cultivated land produc-ing sugarcane, Pelargonium, vanilla, fruit trees, cereals, vege-tables, fodder crops and forest.
On the leeward coastal slopes, between altitudes of 700 and 1200 m with annual rainfall of 1200 to 1800 mm and a marked dry season, Andisols that have formed on ash support rainfed hoed crops, horticultural production (in particular the `rosat' Pelargonium, for perfume) and food crops. Erosion of A hori-zons by water is one of the major problems. As an average value over all the cropped Andisols, soil loss in the rainy season may be 50 t ha71year71, and when cyclones occur, as
much as 200 t ha71year71 has been measured (Bouge©re,
1988).This can lead to abandoning plots.
In areas where the rainfall is better distributed, and when the altitude allows, Andisols can also support plantation or export cash crops. On the windward side of Reunion Island it is sugarcane. Under sugarcane, the soils are well protected from erosion but workability is often determined by the location (slope, accessibility, limited trafficability) and the plots are often not easy to reach with harvest and transport machinery (Perret, 1997).
In the Southern part of Guadeloupe, banana plantations replaced sugarcane in the1970s.Today systems are most com-monly mechanized monocultures, with replanting every 3 or 4 years. Soil fertility is considerably lower than in perennial banana plantations on the leeward coast of the island. Main-taining yields requires frequent applications of pesticide and fertilizer, and the structural state of soil is deteriorating, accompanied by anaerobiosis, necrosis and parasite infesta-tion of rooting systems (Dorel, 1990). The heavy machinery used in replanting accounts for this deterioration.
PHYSICAL PROPE RTI ES OF AN DI SOLS: A B RI E F REVI EW
The study of Andisols began in the 1950s with issues relating to trafficability (Birrell,1952).These issues become critical in the case of cultivation on slopes and/or with heavy rainfall.
The physical components of soil fertility discussed below interact closely with crop emergence, the development and function of rooting systems, the choice in cropping systems and trafficability.
Thixotropy and irreversible desiccation
In volcanic areas in the wet tropics, andic type soils are always found in sequences from brown halloysite soils (Andic Tro-pept) through slightly desaturated (Eutrandept) then desatu-rated Andisols (Dystrandept) to allophanic waterlogged Andisols (Hydrandept) in areas of heavy rainfall (Colmet-Daage & Lagache, 1965; Quantin, 1972; Zebrowski,1975).
Moist andic material, although not rich in lattice clays, contains 80% of fine non-crystalline material. This results in affinity for water and very high porosity (Paterson, 1976; Rousseaux & Warkentin, 1976; Egawa, 1977; Wada & Wada, 1977; Maeda et al., 1977). Despite the high water content, the material is friable and feels moist rather thanwet, but adheres strongly to working parts of machinery. Aggregates show con-siderable structural stability.
Under heavy pressure, the soil abruptly becomes liquid, a phenomenon known as thixotropy. When pressure ceases, the liquid material reverts to its solid state (Perret, 1992). This characteristic directly determines the behaviour of machinery on Andisols leading to limited trafficability, pro-nounced wheel-slip and destruction of upper soil layers and clogging of working parts of the machinery (Wesley, 1973; Maeda et al., 1983).
When a drying threshold is reached, which varies accord-ing to mineral parameters, the hydrated microporous structure of reclaimed Andisols progressively collapses irreversibly. This entails a drop in affinity for water and destruction of clods, accompanied by an increase in the stabi-lity of fine aggregates, and the formation of pseudo-sands (Maeda & Warkentin, 1975). Microporosity diminishes and is replaced in part by inter-particle macroporosity which
Table 1. Chemical and physical characteristics of two Andisols.
Desaturated Andisol (Dystrandept) in food crops in Re¨union,1000 m altitude, 1500 mm rainfall, original parent material: trachytic pyroclastic deposit (15 000 years).
Depth Horizon pHH2O N C P CEC Water content Bulk density
(cm) (%) (%) (ppm) (mmol) (%) (g cm73)
5^15 A 5.8 7.5 8.3 596 16.3 66 0.75
40^60 B 5.7 5.3 7.2 246 10.4 120 0.53
Desaturated Andisol (Dystrandept) under banana in Guadeloupe, 250 m altitude, 3500 mm rainfall, original parent material: andesitic ash deposit (20 000 years).
Depth Horizon pHH2O N C P CEC Water content Bulk density
(cm) (%) (%) (ppm) (mmol) (%) (g cm73)
0^25 A 5.25 5.1 5.6 138 6.1 80 0.8
25^50 B 6.25 1.8 2.6 7 2.2 158 0.5
P: bioavailable phosphorus, by Olsen-Dabin method, CEC: cation exchange capacity, by Hexamin-Cobalt method; C: total carbon, by Anne method; N: total nitrogen by Kjeldahl method.Water content and bulk density measured at field conditions, using in-field core method (200 cm3).
favours accelerated flow of fluids, and mineralization of newly formed organic compounds (Colmet-Daage et al., 1972; Kubota, 1972;Tuncer et al., 1977).
These alterations to the physical environment and water content lead to changes in the microbial population and fauna. Likewise, the transport of the soil by runoff water is facilitated and this triggers erosion.There is a marked struc-tural boundary between the dried-out layer and the moist underlying B horizon, which results in capillary discontinu-ity and problems of root penetration (Dorel,1993).
These processes start as soon as Andisols subjected to a dry season are brought into cultivation. At first they affect the top few centimetres of soil and the subsequent progres-sion depends on the cropping system. Hoeing, which peri-odically denudes the soil and involves redistribution of the upper layer, increases the depth of the soil layer affected, and tilling triggers these processes throughout the depth of culti-vation (Ogawa et al., 1988; Perret, 1992).
Classical soil indicators such as apparent density or total porosity are not sufficient to describe the processes at work in these soils with high water content.
Structural states and cropping systems
In cultivated horizons, various processes are found: several cycles ofdesiccation generate a coarse,stable skeleton from the amorphousmatrixcharacterizedbycontinuousmicrostructure of the B horizon. Biological activity reinforces this tendency to microaggregation and cultivation involves mechanical forces that can re-fractionate these elements. More marked localized mechanical forces (paths, tracks, regular hoeing and ploughing ofupperlayers)then cause these structures generated by desic-cation to evolve towards a powdery fine silty material, that is easilydisplacedbywindandwater.
Field observation of an Andisol profile reveals the marked structural duality between the horizons. In the moist state, the deep B material presents a smooth, continuous fabric with no visible cracks or aggregates. In contrast, the upper horizon is always clearly structured.
When the original A humus layer is preserved, it often comprises very stable aggregates, mainly produced by fauna. If the deepest weathered B horizon occurs at surface level fol-lowing erosion/removal of the A horizon, it has a particulate, finer structure, due to irreversible drying. Aggregation then depends mainly on the degree of desiccation reached and mechanical forces involved.
Clods can have very different characteristics. In the case of undisturbed A humic horizons, the clods are porous cohesive units composed of coarse aggregates. Cultivation gradually reduces this cohesion causing these structures to degrade into particulate aggregates produced by the action of imple-ments (Perret, 1992). In the case of mechanized systems in intensive banana or sugarcane production, tightly packed clods are common, as results of high mechanical pressures (Dorel,1993).
C ROPPI NG SYSTE M S ON AN DI SOLS AN D FE RTI LITY: EXAM PLES
The sustainability of a cropping system can be described using many different indicators: water balance and mineral
budgets, field observation of soil profiles, farm work sche-dule, financial budgets and socioeconomic indicators. The physical properties of Andisols are closely related to agricul-tural practice. On steep uplands, it is the efficiency of erosion control in cropping systems that chiefly determines sustain-ability, but mechanized tillage also causes deterioration of fertility.
Shifting or fixed systems
In traditional shifting Pelargonium cultivation systems in the uplands of Re¨union, composted residues from distillation of the Pelargonium plants are all returned to the land, without any extra mineral fertilizer in the early years. Considerable erosion related to hoeing practices is accompanied by a gra-dual drop in yields and the mineral fertilization that is then introduced has only a palliative effect. Indeed, the deteriora-tion in yields and the fall in work productivity caused by weed encroachment and crop failure lead to abandoning of the fields. New plots are then cleared and cultivated.
Fixed systems, which are taking over from the shifting sys-tems since the end of the 1970s, prove to be catastrophic, since they use the same technology without being able to restore soil fertility by bush fallow.
The land tenancy system known as `colonage', widely prac-tised in Re¨union, constitutes an obstacle to the establishment of anti-erosion measures. Colonage is a kind of annual tenant-farming, in which the farmer shares the produce with the owner of the land, who chooses the cropping, and pro-vides most of the inputs. Fertilizer contributions are small because of cashflow problems that progressively increase. Associated food crops compete with the Pelargonium in the farm work schedule at the time when gaps are replanted (low inputs cause high death rate in Pelargonium, mainly because of soil-borne disease). Weeds grow profusely and farmers attempt control by tillage. For lack of available land, the farmer cannot abandon the plots to clear new ones, and for lack of family labour, he reduces the area under cultivation and runs a very unstable cropping systemwith low productiv-ity in relation to land and labour. Without mineral fertilizer, Pelargonium cropping declines and is replaced by food crops. It no longer provides enough compost to stem erosion, the more so because compost is earmarked for vegetable produc-tion.
Table 2 gives an example of this degradation in Pelargonium cropping systems, through the decrease in overall production of essential oil, as well as through the decrease of land pro-ductivity.
Shifting cultivators depend on farming for only a small part of their family income, and although they are today a marginal category, the numbers are still considerable.
Figure 1 shows a model of the overall soil and fertility degradation process under traditional cropping systems in Re¨union highlands.
Table 2. The decrease in the production and productivity of Pelargonium essential oil in Re¨union (Chastel,1992).
Year 1972 1981 1989
Total production (t) 120 64 16
Average yield{ (kg ha71) 30 23 10
Attempts at intensification
Land reform has encouraged the development of owner-farmed systems, efforts to open-up isolated areas and other land developments in Re¨union. Owners are turning towards market gardening as soon as they have access to water for irrigation, during the dry winter period. Secondary inter-row crops (bean, maize, tomato) are tending to disappear. Manure is provided by introducing beef or goat units onto the farms. These small livestock units also provide mobiliz-able capital and cash flow reserves when times are difficult. Similarly, Pelargonium cultivation is often maintained because it ensures minimum income whatever the weather conditions and whatever the inputs.
In these systems, large regular application of organic man-ures and erosion control measman-ures limit soil deterioration. (e.g. planting in rows at optimal densities, plant-covered ridges with fodder cane or agroforestry).The use of herbicides often enables a reduction in hoeing, and frequently the only mechanization is the knapsack sprayer.
Rotation withvegetable crops (often tomato) and sugarcane is often practised where altitude is suitable, it is seen as a means of soil rehabilitation by the farmers, as confirmed by field observations (Perret et al., 1996b).
Weed control is sometimes not achieved at the end of the wet season, and plants that multiply vegetatively can become invasive (e.g. Phalaris arundinacea, Oxalis sp., Cyperus rotondus). Increased workload and shortage of labour can lead the farmer to resort to ploughing to avoid manual hoeing. Good quality ploughing buries these weeds efficiently, but it is rarely well performed by the contractors. Furthermore, other weed species invade the ploughed land (e.g. Raphanus raphanistrum) and erosion problems are aggravated.
In cropping systems based on Pelargonium and food crops, resorting to tillage is not justified by the needs of the crops. Only tobacco makes good use of deep soil preparation, achieving 10^15% higher yields than with minimum tillage. Pelargonium and food crop yields are generally lower following ploughing, when compared to yields obtained from furrowing or planting by hand in holes (Michellon & Garin, 1985). Thus, the adoption of no-tillage and permanent cover crops seems to be beneficial. Table 3 summarizes changes in soil physical behaviour related to cropping system. These data show increased permeability resulting from plant cover, and that crop rotation cannot by itself restore soil properties.
Young farmers with training are gradually giving up Pelargonium production to specialize in market gardening in
response to grants, the emergence of urban markets and improved communications. The organic matter essential for vegetable production is purchased from penned livestock units at low altitudes, or produced on-farm from cows or goats.
The problem of erosion is often taken into account at field scale (tillage and planting according to contour line, hedges as plot boundaries). But the tillage necessary for vegetable production can cause considerable soil loss in the cyclone season, because of the small crop cover. Minimum tillage and direct drilling, coupled with manure application prove to be effective in such systems (Michellon & Garin, 1985).
Intensive mechanized production
Banana plantations were established on the leeward coast of Guadeloupe at the beginning of this century on young soils on steep slopes. Following the decline of sugarcane in the 1970s, banana cultivation spread over the mechanizable level parts of the windward coasts, on older halloysite soils and on Andisols.
Today, the larger part of the banana plantations on Andisols are replanted every 3^4 years and sometimes more frequently. Replanting requires repeated passes by heavy machinery to destroy the previous plantation and then to pre-pare the soil.The energy required for these operations is con-siderable: whereas only about 1300 kWh are required in halloysitic clays on Andisols as much as 2300 kWh are used during the 15^20 h ha71, (data collected in Guadeloupe;
Perret et al., 1996a).
In some upland areas there are also non-mechanized per-ennial banana plantations. Accounting for only small areas, on some farms, banana is produced in rotation with market garden crops or pineapple.
The impact on rooting systems and soil fertility varies from system to system. In perennial non-mechanized banana, the absence of any mechanized intervention preserves the friable structure of the topsoil, with its high inter-aggregate porosity, and avoids deterioration. The deep horizon (B) has a smoother structure with high tubular porosity.Water and air movements are satisfactory.The roots are healthy and spread down to a depth of 60 cm.
Table 3. Soil surface properties and behaviour under rainfall simulation, according to cropping practices and surface management (Perret et al, 1996b; Perret, 1992)
Soil use and management Ksat{ MWD{ Soil loss½ (mm h71) (mm) (kg h ha71)
Long-term fallow 250a 2.50a 0^0
Pelargonium monoculture on degraded bare 40b 1.10b 208^635 soil (hoed)
Pelargonium=food crop rotation on bare soil 60b 1.11b 10^207 (herbicide)
Pelargonium with kikuyu grass cover 105c 1.41c 0^7 Ksat and MWD: different letters show significant differences between sites, tested by student's t-test at P 0.1.
yKsat: conductivity at saturated soil, in field measurement with disc
infiltrometry.
{MWD: aggregates' mean weight diameter from laboratory experiments. ½Soil loss: average values from measurement with rainfall simulations under
intensities of 45^72 mm h71.
Fig. 1. Degradation processes in traditional agricultural systems growing Pelargonium and food crops on Re¨union.
The repeated forces exerted on soils in mechanized culti-vation, aggravated by high rainfall (up to 4000 mm year71),
lead to soil structure degradation (Table 4), associated with symptoms of root death due to waterlogging. Most of the healthy functional roots are in the topsoil where the structure is friable. Earthing up by hand produces a volume of soil that is better aerated and favours healthy root development at the base of the plant. However the volume concerned is small, and root development stops on the fringes of the mound causing apex necrosis.
Nematodes, mainly Radolphus similis, which are parasitic on the roots of banana trees, cause considerable loss of produc-tion leading to an increase in use of costly nematicides. In intensive monoculture, applications of nematicide are sys-tematic and frequent (3 per year) but provide only limited control of the parasite.
When healthy plant stock are obtained from in vitro cul-ture, very low populations of nematodes are observed for the first 20 months, without any treatment. Incidence of root necrosis is under 20%. From the 20th month, reinfestation of certain plots occurs progressively, accompanied by 50% incidence of necrosis.
Loridat (1989) showed the pathogenic character of Cylin-drocladium sp., a fungus found on banana roots. It is frequently detected on Andisols under monoculture. Rotation with a single-year cycle of pineapple does not eliminate it. In con-trast, it has never been detected on plots with a rotation of 5 to10 years pineapple, or on rotation involving Bracharia spp.
Figure 2 shows a model of the soil and fertility degradation process under mechanized banana cropping systems in Guadeloupe.
To improve the sustainability of these banana plantations, work has been done on these rooting problems (Dorel, 1990).The system developed is based on:
(1) planting healthy micropropagated seedlings in soil that has been cleared of disease by a long rotation;
(2) chemical treatment against nematodes based on regular nematode analyses rather than systematic use of chemi-cals;
(3) increased time intervals between replanting, which reduces frequency of intervention by heavy machinery. Adoption of cropping systems of this nature, which limits physical and pest damage to roots, has been shown to ensure better use of fertilizer, reduced nematode treatment and, increased productivity of banana plantations with reduced costs.
CONC LUSIONS
Andisols have many intrinsic qualities that favour their use for cropping. However, they prove to be very sensitive and their fertility decreases when they are badly managed. The combined effects of slope and high rainfall lead to a very high but variable erosion risk, according to cropping practices. The absence of erosion control measures, along with hoeing in the traditional cropping systems, and the tilling practices in the intensified market gardening systems, have disastrous consequences. Erosion causes deterioration of soils, a decrease in plant vigour and proliferation of weeds.
In industrial banana units, frequent mechanized opera-tions cause deterioration of soil structure, which has severe consequences on yields and production costs.
In these situations, there are increasing problems of weed control, soil-borne disease, erosion and runoff, with more anoxia, necrosis and parasite infestation of rooting systems, reduced infiltration rate and greater trafficability problems. These show up in falling yields, and in increased inputs (mechanical preparation and pest treatment in particular), and greater risks, i.e. less scope and less security in the choice and implementation of cropping systems, and likewise in achieving yield.
The examples developed here underline the interplay between cropping systems and the physical characteristics of Andisols, and also the permanent interaction between the potential of the soil, husbandry, and sustainability of the sys-tems.
The emergence, or the abandonment, of farming practices enabling soil protection are dependent on socioeconomic fac-tors that are often outside the control of the farmers them-selves (land tenure, land distribution, management of labour, economic environment, markets, etc.). The promotion and spread of relevant innovative cropping systems requires those concerned at local level to be familiar with these factors, even if they cannot gain control over them.
The most recent research shows that the deterioration of the structural state and the fertility of Andisols can be reversed by fairly straightforward practices: direct drilling, permanent plant cover during cropping, agroforest hedging, manure and organic matter application (Perret etal., 1996b), the use of Bra-charia fallowin rotationwithbanana(Dorel,1993).
Current research programmes in Re¨union and Guade-loupe consist in building on farmers'experience and husban-dry, at the same time as testing and promoting alternative technologies including use of cover crops, agroforestry and planting of more healthy plant material.
Fig. 2. Degradationprocesses in intensivebananaproduction inGuadeloupe. Table 4. Soil surface characteristics according to banana cropping practices:
data range (min^max) or average from field experiments (Dorel, unpublished data).
Cropping system Ksat{ (mm h71) Macroporosity{
Mechanized cropping system, frequent 36 ^ 295 25 replanting
Non-mechanized, perennial plantation 220 ^ 480 85
{Ksat conductivity at saturated soil, in field measurement with disc
infiltrometry.
{Macroporosity % of overall porosity, laboratory measurement from
RE FE RE NC ES
BIRRELL, K.S. 1952. Some physical properties of New Zealand volcanic ash
soils. In: Proceedings, 1stConference on Soil Mechanics and Foundation
Engineer-ing, Melbourne, pp. 30^34.
BOUGE©RE, J.1988. ApercËu sur l'e¨rodibilite¨ des andosols cultive¨s a© La Re¨union. In: Les andosols de l'õÃle de La Re¨union, pre¨paration d'un programme de recherches pluridisciplinaires, CIRAD-CNRS-INRA-ORSTOM-Universite¨, Se¨mi-naire de Saint Denis, 24 May^1 June1988, pp. 155^162.
CHASTEL, J.M. 1992. La filie©re ge¨ranium a© la Re¨union: situation et
perspec-tives. In: Le ge¨ranium rosat a© la Re¨union CIRAD e¨dit., Graphica, Saint Denis, pp. 7^14.
COLMET-DAAGE, F. & LAGACHE, P. 1965. Caracte¨ristiques de quelques groupes de sols de¨rive¨s de roches volcaniques aux Antilles francËaises. Cahiers ORSTOM, se¨rie Pe¨dologie III, 91^121.
COLMET-DAAGE, F. GAUTHEYROU, J. & M., DEKIMPE, C. & FUSIL, G.1972. Dispersion et e¨tude des fractions fines de sols a© allophanes des Antilles et d'Ame¨rique Latine. le©repartie: La dispersion. Cahiers ORSTOM, se¨rie
Pe¨dologie X, 169^191.
DOREL, M. 1990. Influence du milieu et des techniques culturales sur la productivite¨ des bananeraies de Guadeloupe. Enqueªte-diagnostic. Fruits. 45, 237^244.
DOREL, M.1993.Travail du sol en bananeraie: cas des andosols. Fruits 48, 77^
82.
EGAWA,T. 1977. Properties of soils derived from volcanic ash. In: Soils derived
from volcanic ash in Japan (eds Y. Ishizuka & C.A. Black) CIMMYT, Mexico, pp. 10^63.
KIMBLE, J.M. 1998. Properties and classification of andisols in soil taxon-omy: U.S. classification of andisols. In: Proceedings COST 622 meeting, Soil Resources of European Volcanic Systems, Iceland, 5^10 July1998, pp. 20^25. KUBOTA,T.1972. Aggregate-formation of allophanic soils: effect of drying on
the dispersion of the soils. Soil Science and Plant Nutrition 18, 79^87. LORIDAT, P. 1989. Etude de la microflore fongique et des ne¨matodes associe¨s
aux ne¨croses de l'appareil souterrain du bananier en Martinique: mise en e¨vidence du pouvoir pathoge©ne du genre Cylindrocladium. Fruits 44, 587^ 598.
MAEDA, T. & WARKENTIN, B.P. 1975.Void changes in allophane soils deter-mining water retention and transmission. Soil Science Society of America Pro-ceedings 39, 398^403.
MAEDA, T. TAKENAKA, H. & WARKENTIN, B.P. 1977. Physical properties of allophane soils. Advances in Agronomy 29, 229^264.
MAEDA, T. SOMA, K. & WARKENTIN, B.P. 1983. Physical and engineering characteristics of volcanic ash soils in Japan compared with those in other countries. Irrigation Engineeringand Rural Planning 3, 16^31.
MICHELLON, R. & GARIN, P.1985. Recherche-Syste©me dans les Hauts de l'ou-est; Synthe©se des re¨sultats obtenus: le ge¨ranium, le haricot, la pomme de terre, le tabac, le maõÈs, les fruitiers tempe¨re¨s. In: Bilan de la Recherche-Sys-te©me dans les Hauts de l'ouest. Journe¨es du 25^27 November 1985, CIRAD Re¨union, pp. 129^201.
OGAWA, K.TAKEUCHI, Y. & KATAYAMA, M.1988. Influence of minimum til-lage on soil properties and crop yields in gleyic andosol. Research Bulletin of the Hokkaido National Agricultural Experiment Station 150, 57^90.
PATERSON, E.1976. Specific surface area and pore structure of allophanic soil clays. Clay Minerals 12, 1^9.
PERRET, S. 1992. Etude des proprie¨te¨s physiques, hydriques et me¨caniques de sols
andiques de la Re¨union. Facteurs naturels et anthropiques d'e¨volution des horizons culturaux, implications agronomiques et e¨cologiques. PhD Thesis, Ecole Natio-nale Supe¨rieure Agronomique de Montpellier.
PERRET, S. PIROT, R. & DOREL, M. 1996a. Pre¨paration me¨canise¨edes sols en
bana-neraie: itine¨raires techniques et performances des mate¨riels sur deux sols volcaniques de Guadeloupe. Notes de recherches, no. 2/96, CIRAD, 7pp.
PERRET, S. MICHELLON, R.TASSIN, J. & BOYER, J. 1996b. Soil rehabilitation and erosion control through agro-ecological practices on Reunion Island (French Overseas Department, Indian Ocean). Agriculture, Ecosystems and Environment 59, 149^157.
PERRET, S. 1997. Trafficabilite¨ et gestion des ope¨rations me¨canise¨es: e¨tudes
expe¨rimentales, mode¨les et formes d'aide a¨ la de¨cision en culture de canne a© sucre. In: Le travail du sol dans les syste©mes me¨canise¨s tropicaux (eds R. Pirot, S. Perret & H. Manichon), Actes du colloque, 11^12 Septembre 1996, Montpellier, CIRAD, collection Colloques, 160pp.
QUANTIN, P. 1972. Les Andosols. Revue bibliographique des connaissances actuelles. Cahiers de l'ORSTOM, se¨rie Pe¨dologie X, 273^301.
RAUNET, M. 1991. Le milieu physique et les sols del'õÃle de La Re¨union. Conse¨quences pourla mise en valeuragricole. CIRAD/Re¨gion Re¨union, 438pp.
ROUSSEAUX, J.M. & WARKENTIN, B.P. 1976. Surface properties and forces
holding water in allophane soils. Soil Science Society of America Journal 40, 446^451.
TUNCER, E.R. LOHNES, R.A. & DEMIREL, T. 1977. Dessiccation of soils derived from volcanic ash. Transportation Research Records 642, 44^49. WADA, S.I. & WADA, K.1977. Density and structure of allophane. Clay minerals
12, 289^298.
WESLEY, L.D. 1973. Some basic engineering properties of halloysite and allophane clays in Java, Indonesia. Geotechnique 23, 471^494.
ZEBROWSKI, C.1975. Etude d'une climatose¨quence dans l'õÃle de La Re¨union.
Cahiers ORSTOM, se¨rie Pe¨dologie XIII, 255^278.
Received September1998, accepted after revision January1999.
